Our group has taken a "holistic view" in the study of orogenic belts, where
we are interested in the complete view of orogenic systems, from prograde
metamorphism and subduction to ultra-high-pressures (UHP) to magma genesis and
the timescales of volcanic evolution. Such an approach requires an
intimate interweaving of petrology (metamorphic and igneous), field work,
structure/tectonics, and geochronology using multiple isotopic systems. No
one tool or approach can solve all problems. There are two main projects
that we are currently pursuing:

1. HP & UHP Metamorphism: A study of the
prograde and retrograde tectonothermal evolution of the Western Alps,
Switzerland and Italy.

2. Timescales of magmatic processes: Studies
of subduction-related volcanism over the 105 year scale.

1. HP & UHP-Metamorphism and tectonothermal evolution of the Western Alps, Switzerland and Italy

The Alps remain a classic laboratory for studying the tectonics of
continent-continent collision. The Zermatt-Saas ophiolite complex of the Western Alps is a remnant of the Tethys Ocean that was subjected to high
(HP) and ultra-high pressure (UHP) metamorphism, followed by rapid exhumation during the
Alpine Orogeny. Despite over a century of structural studies and decades of
geochronological research in this region, the age and duration of HP to UHP
metamorphism, the maximum pressures and temperatures the Zermatt-Saas ophiolite
and structurally underlying basement nappes experienced, and the rate at which
these units were buried and exhumed remain unresolved problems. These
questions continue to be hotly debated, and are critical to understanding of
the geodynamic processes in orogenic belts. We have focused on the Alps
because this orogenic belt is young enough so that temporal relations may be
worked out in detail, the exposures are superb, the maximum temperatures were
relatively cool, below the blocking temperatures of several geochronometers, and
the prior geologic work provides a excellent framework upon which to build new
studies. In addition to work on the Zermatt-Saas complex, we have extended
our work to the Monte Rosa nappe (a fragment of European basement) and the Sesia
nappe (a fragment of African crust).

The 3D topographic/bathymetric render above (left) looks down on the Alps
(lower part of image) from the NW into the Mediterranean Sea and northern Africa
(upper part of image). Prior to opening of the Mediterranean, the Apulian
plate ("Africa") was thrust onto Europe in the late Mesozoic to early Cenozoic,
forming the HP-UHP terranes of the Alps, as well as Greece (upper left of
image).

The map above (right) illustrates the major units of the Western Alps.
Working from lower left (SE) to upper right (NW), the light yellow is the Po
Plain, followed by the orangish/tan units of the Apulian plate ("Africa"), the
dark blue of the Zermatt-Saas ophiolite, and the pink/salmon units of European
basement massifs. The geology is complex, where, for example, the upper
part of the Matterhorn (see book inset above, and photo below) is a fragment of
Africa, and the lower part is the Jurassic Tethys oceanic crust (Zermatt-Saas
ophiolite). Units of the ophiolite complex are exposed in the lower parts
of the photo below, including pillow lavas and serpentinite bodies (foreground
knob on left in photo below).

HP and UHP metamorphism is recorded in the eclogites of basaltic protoliths,
as seen in the photo below. The garnets are obvious, and these represent a
surprisingly long prograde metamorphic history of garnet growth during
subduction of the oceanic crust. Eclogites are common in both the
Zermatt-Saas complex, as well as older units such as the Sesia nappe that lies
to the SE and reflects sections of African continental margin.

The key to understanding the prograde history of these
terranes lies in the elemental zonations recorded in garnet. The
image on the left is a Yttrium X-ray map of a garnet that has been
sectioned precisely through the center as part of a parallel 3D X-ray
tomographic study (courtesy of Lukas Baumgartner, Univ. Lausanne).
Yttrium, which may be used as a proxy for Lu, is concentrated in the core
during the initial phase of garnet growth. The high-Y band partway
out reflects the dynamics of garnet growth and diffusional transport of Y
to the garnet, which is then followed by a second rimward depletion.
The large variations in Y contents are mirrored by variations in Lu
(determined separately by laser ablation ICP-MS), which predicts strong
core-to rim zonation in ages that would be determined by the 176Lu-176Hf
geochronometer. The question is, how long did it take to grow this
garnet during the prograde subduction path?

Through combining careful field observations with petrologic and
geochronological studies, we have assembled a preliminary history of the
Zermatt-Saas ophiolite, which is shown in the figure below. Garnet growth
models, developed by our collaborator
Lukas Baumgartner
at the University of Lausanne, constrain the interplay of thermodynamic
stability of garnet in P-T space with zonations in Lu/Hf and Sm/Nd ratios as
determined by elemental diffusion and equilibrium partitioning; these are then
fed into models for the 147Sm-143Nd and 176Lu-176Hf
geochronological systems. The strong core-to-rim zonation in Lu/Hf ratios
of garnet indicate that 176Lu-176Hf ages should be skewed
to the early periods of garnet growth as compared to the 147Sm-143Nd
ages, and indeed this is the case (see figure below; from Lapen et al., 2003).
These results suggest that prograde garnet growth occurred over perhaps 15 m.y.
or more,
which is on the order of plate velocities that have been estimated for
subduction of Tethys crust during the Alpine Orogeny. For comparison, U-Pb
zircon geochronology, a very common geochronological system, places no
constraint on the age of metamorphism at specific P-T conditions, and scatter
over the prograde interval (see figure below). The uplift path, which was
very rapid, is constrained by the 87Rb-87Sr geochronological system on
greenschist-facies overprints, as well as other constraints, such as zircon
fission-track ages (not shown)

We are continuing our work on the Zermatt-Saas complex, but are also
extending new work into the Sesia nappe, which lies to the SE and is the
earliest nappe unit to have been subducted to HP (or greater) conditions during
the Alpine Orogeny. Eclogites do occur in the Sesia (photo below, looking
north into Switzerland), although granitic lithologies are most abundant.
The Sesia reflects the rifted margin of Africa that collided with the European
margin during the Alpine Orogeny.

2. Timescales of Arc Magmatic Processes in the Aleutian,
Cascade, and Andes arcs

A frontier in petrology and geochemistry
involves quantifying timescales of magmatic processes. Accurate
temporal information is critical to understanding how potentially
dangerous arc volcanoes are built, how fast they grow, and how they
eventually fail through catastrophic eruption or sector collapse.
Yet we know little about the long-term variability of eruptive fluxes from
arc volcanoes, associated timescales of magma crystallization and
differentiation, relations between magma input vs. output events, and the
role that crustal thickness plays in determining how long a particular
magma takes to transit the crust and erupt. We are addressing these
issues at several carefully selected volcanoes by combining: (1)
stratigraphy, petrography and geochemistry of lava flows and pyroclastic
deposits, (2) precise 40Ar/39Ar dating to determine
an eruptive chronology, and (3) 238U-230Th isotope
measurements of phenocrysts and host rocks to constrain the timing of
crystallization relative to eruption and to fingerprint closed and
open-system magmatic processes. The half life of 230Th,
for example, is ideal for reconstructing the history of magmatic and
volcanic events shaping a stratovolcano over the past 250,000 years.

We have been working on three volcanic arcs, each of which has unique
characteristics. The Aleutian arc in SW Alaska (3D image in above
left) is a relatively primitive arc that has significant east-west
variations in the nature of the subducted slab, including age and
occurrence of fracture zones. Work has focused on Kanaga, Roundhead,
Seguam, and Shishaldin volcanoes, each of which has a different eruptive
style and composition. Work in the Aleutians has been in
collaboration with
Prof. Brad
Singer in our Department, and has involved 40Ar/39Ar
geochronology, as well as Sr, Nd, Hf, and Pb isotope tracer work, and
238U-230Th geochronology.

We have also been working on several volcanoes of the Cascade arc in
Washington, Oregon, and California. In the 3D geologic map on the
left, the modern stratovolcanoes cones of the Cascade arc are shown in
pink, and we have been working on Mt. Adams, Crater Lake, Mt. Shasta, and
Mt. Lassen with collaborators at the U.S. Geological Survey. The
Cascades may be considered one of the most "primitive" of the continental
volcanic arcs today. This work has included Sr, Nd, and Pb isotope
work, as well as innovative new work using the 187Re-187Os
isotope system as a unique tracer of primitive basaltic crust at depth
that cannot be "seen" using traditional isotope systems such as O, Sr, Nd,
Hf, and Pb; this work has been in collaboration with Steve Shirey at
Carnegie-DTM.

Finally, also in collaboration with
Prof. Brad
Singer in our Department, we have been working on several volcanoes in
the Andes arc of South America (3D image at left). This includes a
study of the contrasting evolution of Parinacota and Puyehue volcanoes,
which erupted through thick and thin continental crust, respectively.
This work has involved 40Ar/39Ar geochronology, as
well as Sr, Nd, Hf, and Pb isotope tracer work, and 238U-230Th
geochronology. It is anticipated that Parinacota will have a longer
residence time for magmas due to the great thickness of the underlying
crust, whereas the relatively primitive Puyehue volcano may have involved
much shorter magma ascent and crustal differentiation times.

An example of applying the 238U-230Th isotope system
to studying volcanic evolution is shown below, where detailed stratigraphic
studies and 40Ar/39Ar geochronology allows us to correct
for in situ 238U decay, producing highly precise initial 230Th/232Th
ratios (see Jicha et al., 2004; 2005). This work has been led by
Prof. Brad
Singer and his students in our Department. At Seguam volcano in the
Aleutians, the increase in initial 230Th/232Th ratios
over the last 140,000 years is interpreted to reflect a closed-system
evolution by 238U decay, a remarkable conclusion considering the expectation
that this subduction-related system would be open to input from mantle melts
over this time. Following collapse of the Wilcox caldera 9,000 years
ago, the initial 230Th/232Th ratios decrease up through
the recent eruptions (1993 and 1997), suggesting mixing with new,
mantle-derived magmas.